Currently, the most common forms of ultrasound imaging systems generate two-dimensional images of a cross-section of the subject of interest by electronically scanning an assembly of piezoelectric elements in either linear format or sector format. FIGS. 1A and 1B illustrate the naming conventions used in ultrasound engineering. FIG. 1A illustrates the conventions of orientation and direction. As shown in FIG. 1A, the transducer 100 is typically made up of multiple transducer elements 110. The transducer elements 110 are oriented such that their lengths are along the elevation axis, and their widths are along the azimuth axis. The transducer elements 110 are adjacent to one another along the azimuth axis. FIG. 1B illustrates the linear 210 and sector 220 image formats generated by a typical ultrasound system. As shown in FIG. 1B, in linear format 210 scanning, time delays between transducer elements are used to focus the ultrasound beam in the image plane. Also shown in FIG. 1B, in sector format 220 scanning, time delays between transducer elements are used both to focus the ultrasound beam and to steer it.
Ultrasound systems that generate three-dimensional (3-D) images of the subject of interest are also available. Most of the commercially available systems form three-dimensional images from multiple two-dimensional (2-D) slices taken by a mechanically translating or rotating probe. An example of such a system is General Electric's Voluson 730, which has its origins the work of Kretz in Austria. U.S. Pat. No. 4,341,120, issued in 1982, describes a multi-element probe that is electronically scanned in the azimuth direction, but is mechanically moved to capture image slices in the elevation direction.
Mechanical translation suffers from several disadvantages, among them cost, reliability, and mechanical jitter. The resolution of the reconstruction of the image in the elevation direction is a function of the slice thickness of the elevation profile of the transducer, as well as of the positioning accuracy of the mechanical translation scheme.
Other approaches to 3-D imaging are also known in the art. Systems based on two dimensional transducer arrays are taught in, for example, U.S. Pat. Nos. 4,694,434, 5,229,933 and 6,126,602. One disadvantage of 3-D imaging systems based on 2-D transducer arrays is that the interconnect that connects an individual transducer element to it's controlling circuitry can be difficult and expensive to design and manufacture.
Given the acceptance of the mechanical scanning format, and the system infrastructure already available, it would be advantageous to provide for probes that offer the three-dimensional imaging capabilities of mechanical scanning and improvements thereon without the detriments of mechanical scanning. Thus, a probe capable of being electronically scanned in the elevation direction, in a manner analogous to mechanical translation, is desirable.
Recently, capacitive microfabricated ultrasonic transducers (cMUTs) have been demonstrated to be viable alternatives to piezoelectric transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes capacitive microfabricated transducers capable of competitive acoustic performance with piezoelectric transducers.
Several inventors have recently described aspects of controlling MUTs with bias voltage. In commonly owned U.S. Pat. No. 7,087,023, the use of bias polarity patterns to control both the phase profile and the aperture in elevation is taught. Bias polarity provides aperture control that is equally effective in transmit and in receive. In commonly owned pending U.S. patent application Ser. No. 10/819,094 to Panda et al., methods of combining bias polarity patterns and multiple firings are taught which enable the cancellation of transducer-emitted harmonics and optimized beam profile control, among other advantages. Savord et al., in U.S. Pat. No. 6,381,197 describe elevation apodization, and elevation focusing by time-based expansion of the receive aperture. In published U.S. application 2003/0048698, Barnes et al. describe a method and system providing bias control of cMUT sub-elements. None of these references teaches or claims specific structures or methods directed to 3-D imaging.
It has been realized by the present inventors that a transducer array with a relatively large elevation dimension and bias control of the elevation aperture in space and time confers the same benefits of mechanical translation, except that image cross-sections are electronically rather than mechanically scanned. It has been further realized by the present inventors that elevation bias control in combination with convex curvature in elevation increases the volume interrogated by the electronic scanning, thus improving field of view. In U.S. patent applications Ser. Nos. 09/435,324 and 10/367,106 Ladabaum et al. teach various structures and methods of curvilinear microfabricated ultrasonic transducers. Further still, the present inventors realized that a fixed mechanical lens for elevation may not be compatible with an electronically scanned aperture, but that Fresnel focusing of the elevation section can be used to improve the elevation focus. Yet another advantage of electronic translation of the elevation aperture is that the accuracy of the position of the elevation slice can be controlled to approximately 100 microns, so that multiple slices can be used to improve 2-D images.
Thus, what is needed is an ultrasonic transducer, system, and method of control, characterized by a readily-manufacturable interconnect scheme, capable of capturing multiple image slices tightly spaced in elevation to form a 3-D image with an adequate field of view, such that mechanical motion of the transducer elements is not needed. The present invention provides such a transducer, system, and method.